This invention broadly relates to a new coating composition and to a method of applying said coating composition to various enclosures to protect electronic equipment from electromagnetic interference (EMI). More particularly this invention relates to a new copper dispersion coating composition usable as a copper shielding coating that maintains high electrical conductivity even after exposure to heat, humidity and/or salt spray; and thereby, it eliminates many of the shortcomings of other copper coatings that have been attempted for this purpose. The new coating composition of this invention may be applied as a one-coat air dry system that is spray applied and needs no protective overcoat. It also of course may be applied by other means such as brushing, dipping or the like.
The state of the art is indicated by the following: U.S. Pat. Nos. 2,750,303; 2,980,719; 3,074,818; 3,142,814; 3,382,203; 3,998,993; 4,123,562; 2,750,307; 3,501,353; 3,532,528; 3,653,931; 3,867,738; 3,867,739; 3,716,427; 3,060,062; German Pat. No. 1,941,328; and Acheson Colloids Co. product data sheets on Electrodag 112, 414, 424, and 433.
It has been known that copper particles dispersed in a binder resin material or solution could be used to make an electrically conductive coating. However all of the prior coatings of this type have had the major deficiency that during storage or during usage, the copper oxidizes and the electrical properties changed such that there was a detrimental effect. Under many usage conditions this electrical property change is of such magnitude that the formerly conductive coating becomes an insulating coating. Attempts to overcome this change of electrical properties have called for replacement of the copper conductive particles with the more expensive noble-metals such as silver, gold and the platinum group, all of which greatly and significantly increase the cost of the electrically conductive coating. Thus a copper dispersion coating has long been sought after, which when applied in film form to a substrate, would provide good electrical conductance properties even after exposure to elevated temperatures for substantial time periods.
Another proposed solution which has been investigated in the past is the treatment of the copper particle to prevent its oxidation and subsequent change in electrical properties. Some of the solutions that have been suggested are treatment of the copper with chemicals such as nitrosobenzene, benzotriazole, chromium salts, silicates, and the like. Other treatments suggested have included high molecular weight alcohols and stearates. These attempted solutions have been ineffective or temporary at best. In addition many of these surface treatments resulted in the copper particle becoming an electrical insulator rather than the desired electrical conductor.
Accordingly a main object of this invention is to provide a new coating composition containing finely particulated copper particles and which composition is suitable for use in forming applied coatings having very good electrical conductance properties.
Another important object of this invention is to provide a novel method of forming EMI shielding coatings on enclosures for electronic equipment.
Another object of the invention is to provide a new copper coating composition which when applied as a coating maintains high electrical conductivity even after exposure to heat, humidity or salt spray.
Another object of the invention is to provide an environmentally stable air dry copper coating.
Another object of the invention is to provide a new copper coating composition which provides controlled electrical properties.
Another object of the invention is to provide a novel method of protecting electronic equipment from electromagnetic interference (EMI) by applying a copper shielding coating to enclosures for the equipment.
Other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims.